The present invention relates generally to wireless communication systems and, in particular. to antenna arrays used in wireless communication systems.
Sectorization of cells is a well-known technique for reusing logical channels in order to enhance the capacity of wireless communication systems. FIG. 1 depicts a conventional sectorized cell 10 in accordance with the prior art. Cell 10 includes three 120xc2x0 sectors A, B, C, and has an associated base station 12 for providing wireless communication services to mobile-telephones within cell 10. Base station 12 includes, for each sector, a set of base station radios 13, converters 15, and an antenna configuration 14 comprising of two antenna elements 16-1, 16-2. The set of base station radios 13 performs digital base-band signal processing and provides wireless communication services to mobile-telephones in its sector over associated antenna elements 16-1, 16-2. Converters 15 are connect to base station radios 13, and include an D/A-A/D converter and base band/RF-RF/base band converter for converting digital base band signals outputted from base station radios 13 into analog RF signals for transmission over antenna configuration 14, and vice-versa for analog RF signals received over antenna configuration 14. Each antenna element 16-1, 16-2 is connected to a converter 15 via a separate cable, wire or optical fiber, not shown, and produces a beam of approximately 120xc2x0 for providing wireless communication coverage (or beam coverage) to mobile-telephones within its associated sector.
The beams produced by antenna elements 16-1, 16-2 are non-variable or fixed beamwidths and, thus, the associated sectors, in effect, are fixed in size. Fixed size sectors are undesirable because traffic distribution and loading patterns may not be uniform across each sector resulting in inefficient utilization of base station radio resources. For example, the number of mobile-telephones in one sector may exceed the capacity (or capability to process signals being transmitted to and from the mobile-telephones) of the associated set of base station radios 13, whereas the number of mobile-telephones in another sector may not exceed the capacity of the associated set of base station radios 13 resulting in unused excess capacity. Or the loading pattern at one time of day may differ from the loading pattern at another time of day due to, for example, commuter traffic resulting in inadequate base station radio resources in one sector and excess base station radio resources in another sector.
To overcome these problems associated with fixed size sectors, adaptive or variable sectorization has been proposed. Adaptive sectorization allows for sector sizes to be adjusted by varying the associated beam coverage. FIG. 2 depicts an adaptive sectorized cell 20 in accordance with the prior art. Cell 20 includes three sectors A, B, C, and has an associated base station 22 for providing wireless communication services to mobile-telephones within cell 20. FIG. 3 depicts a more detailed illustration of base station 22, which includes a set of base station radios 23 per sector, butler matrices 25, converters 21 and an antenna configuration 24. Butler matrices 25 are connected to base station radios 23 for shifting phases of digital base band signals outputted by base station radios 23 to obtain phase shifted digital base band signals. Converters 21 are connected to butler matrices 25 for converting the phase shifted digital base band signals into phase shifted analog RF signals for transmission over antenna configuration 24.
Antenna configuration 24 comprising of an antenna array 27 per butler matrix 25, wherein each antenna array 27 includes four antenna elements 26. Each antenna element 26 in antenna array 27 is connected by a separate cable, wire or optical fiber to converters 21, and produces a beam of approximately 120xc2x0. Thus, twelve cables are required for connecting the twelve antenna elements to converters 21. The 120xc2x0 beams produced by antenna elements 26 of antenna array 27 are combined and manipulated via associated butler matrix 25 to produce four 30xc2x0 beams over which downlink signals may be transmitted, wherein a downlink signal intended for a particular mobile-telephone is only transmitted over the 30xc2x0 beam covering the area in which that mobile-telephones is currently positioned.
Each set of base station radios 23 has an associated set of 30xc2x0 beams (hereinafter referred to as xe2x80x9cbeam setxe2x80x9d). The number of 30xc2x0 beams in each beam set determines the size of each sector A, B, C. Accordingly, the size of each sector A, B, C may be adjusted by varying the number of 30xc2x0 beams in the associated beam set. Suppose each beam set initially includes four adjacent 30xc2x0 beams, thus, each sector A, B, C had a size corresponding to 120xc2x0. If the traffic in sector A of cell 20 exceeds the capacity of the associated set of base station radios, sector A may be adjusted to correspond to the coverage area of three adjacent 30xc2x0 beams to reduce the load of sector A, and either sector B or C may be adjusted to correspond to the coverage area of five adjacent 30xc2x0 beams to increase its load if there exist unused base station radio resources in that sector.
The architecture of cell 20, however, would not be easy to implement in wireless communication systems based on Code Division Multiple Access (CDMA) techniques because a pilot signal is required to be transmitted along with other downlink signals such that coherent demodulation of downlink signals can be performed at the mobile-telephones. If only one antenna element 26 in antenna array 27 is used to transmit the pilot signal, the pilot signal will be transmitted over a 120xc2x0 beam formed by that antenna element 26 and, thus, a phase difference may exist between the pilot signal and downlink signals transmitted over the 30xc2x0 beams making it difficult to coherently demodulate such downlink signals using the pilot signal. Alternately, if a pilot signal is transmitted over every antenna element 26 in antenna array 27, the pilot signal will only be transmitted over a resulting 30xc2x0 beam formed by all the antenna elements 26 through butler matrix 25 and, thus, the pilot signal cannot be used to demodulate downlink signals transmitted over other 30xc2x0 beams. Accordingly, there exists a need for an adaptive sectorization technique that would amiable to CDMA environments.
The present invention is an adaptive sectorization technique that is amiable to CDMA environments using an antenna configuration and a phase shifter network that can form a variable width beam over which a pilot signal and other downlink signals may be transmitted. The antenna configuration having at least an antenna sub-array with two or more antenna elements. The phase shifter network having a plurality of switches for adjusting beam width by directing signals to be transmitted over one or more of the antenna elements, and phase shifters for shifting phases of the signals to be transmitted over the one or more antenna elements. In one embodiment, the phase shifter network is at the RF front end allowing for phase adjustment at the RF front end instead of at digital base band signal processing. Adjusting the phases at the RF front end enables transmission of the pilot signal and downlink signals in a same beam pattern generated by one of the antenna sub-arrays in the antenna configuration, thereby eliminating pilot error resulting when the pilot signal and downlink signals are transmitted over different beams.